1. Technical Field of the Invention
The present invention relates to high-speed precision laser processing of target materials without damaging the region surrounding the cut or modified portion. More specifically, the present invention relates to a pulsed laser micromachining method to ablate or modify a portion of a structure comprised of single or multi-layer thin films. The pulsed laser beam vaporizes a portion of a layered structure comprised of metal, semiconductor material, organic material and/or dielectric material. Electrically conductive thin film links (e.g., copper, aluminum or platinum) and resistive films (e.g., nichrome, tantalum nitride, cesium silicide or silicon chromide) can be modified using the pulsed laser beam.
In an embodiment of the present invention, the metal interconnections in a LSI circuit or the metal links in a memory device (e.g., DRAM and SRAM) can be precisely severed, without damage to the layer beneath the severed portion or collateral damage to the region of the severed portion. In another embodiment of the present invention, micro- and nano-scale semiconductor devices, optical devices and microelectromechanical (MEMS) devices can be machined. In another embodiment of the present invention, biological tissues, artificial tissues and synthetic tissues can be ablated, incised, removed or fused with great precision.
2. Description of the Related Art
The demand for producing small scale features in MEMS, semiconductor, biomedical and other industries has led to the development of very precise laser material processing methods. The precise tuning and manipulation of laser pulse parameters is one method of achieving the precise control required for small scale feature manufacturing.
In some situations, it is crucial that only the region of interest in the processed material is modified, without damage to the surrounding region and/or without change to the physical, mechanical and/or chemical properties of the surrounding region. In addition, the surrounding region must remain in physical and chemical coherency with the region of interest after the laser irradiation so as to avoid induced cracks or defects in the surrounding region. Careful manipulation of laser parameters is necessary to achieve such results.
The fundamental mechanism of laser-matter interaction depends on basic laser parameters, such as pulse energy, pulse width, wavelength, pulse shape and polarization. It is well known that ultrashort pulse lasers that deliver laser pulses having pulse width of less than 100 picoseconds remove material more precisely and cleanly than laser pulses having longer pulse widths (i.e., nanoseconds). It is also known that the material breakdown threshold fluence is more precisely defined with pulse widths in the femtosecond and picosecond range, as compared to longer pulse widths. Only the laser energy above the threshold fluence contributes to the damage. In addition, most of the laser energy dissipation is confined to the irradiated region, minimizing thermal, physical and chemical damage in depth and collateral dimensions.
Breakdown or damage caused by a laser pulse includes any changes caused by electronic change, structural change and/or disintegration from the normal state of material at the ambient temperature. These changes can include dielectric breakdown, plasma formation, ablation, melting and vaporization during laser material interaction. Since ultrashort laser pulses offer a precise control of ablation threshold, precise control of the amount of material removed during materials processing is achieved with minimal debris and heat affected zone.
FIG. 1A is a schematic diagram that depicts a portion of a memory link 12 in a memory chip. FIGS. 1B and 1C are cross-sectional views of the memory link 12. The memory link 12 comprises conductive materials such as metal, polysilicon and polysilicide. The memory link 12 adheres to the passivation layer 11 (e.g., SiO2) on a silicon substrate 10. The passivation layer 11 thickness is typically 0.01 to 5 microns. The memory link 12 is interconnected with other portions of the memory chip by an electrical contact 13. Typically, the memory link 12 is 0.01 to 3 microns thick and 0.1 to 3.0 microns wide. Each link is spaced apart from each other link by about 1 to 5 microns on the silicon wafer. The typical metal link materials are copper, aluminum, gold, silver, nickel, platinum, titanium and tungsten. Other electrically conductive materials are also used.
Laser spot 14 covers the link 12 and preferably, the laser spot size is 10 to 50 percent larger than the width of the link. The link 12 has to be cut without damage to the surrounding and underlying regions. Laser pulse shape, repetition rate and wavelength have to be designed so as not to cause any damage to the vicinity of the metal link.
Conventional memory link cuts employed a single pulse of a nanosecond pulse width in the IR wavelength range. U.S. Pat. No. 5,265,114 and U.S. Pat. No. 5,569,398 emphasize the importance of wavelength, where an infrared laser beam in the wavelength range of 1.2-3.0 microns is utilized. In this wavelength range, silicon is essentially transparent and absorption contrast of the link material and the silicon is maximized. This results in the selective vaporization of metal links on top of the silicon and reduction of damage to the underlying silicon wafer.
U.S. Pat. No. 5,208,437 discloses a sub-picosecond pulse for cutting aluminum interconnects without damaging the vicinity of the cut portion. The laser pulse passes away before melting of the first layer of metal starts and does not reach the layer beneath the first layer of metal.
U.S. Pat. No. 6,574,250 discloses a train of pulses in a burst of duration shorter than 1000 nanoseconds, or more preferably, 300 nanoseconds. Each set of pulse trains contains short pulses of 100 femtoseconds to 30 nanoseconds. The number of short pulses in a burst and energy of each pulse are selected so as to cleanly remove the bottom of the cut link.
U.S. Reissue Pat. No. RE37585E discloses a laser pulse that has a pulse width less than the value determined by the distinct change in the relationship of fluence breakdown threshold energy versus laser beam pulse width.
U.S. Pat. No. 6,281,471 B1 discloses a square shaped pulse of specific range of rise time, duration and fall time. The rise time of about one nanosecond is fast enough to couple the laser energy to the target materials. The pulse duration is about 2-10 nanoseconds and long enough to process the target materials. The fall time is a few nanoseconds and is rapid enough to avoid undesirable thermal effect on the structure.